This invention relates to digital color image processing and, more particularly, to correcting for chrominance interpolation artifacts.
With the advent of digital cameras, it is becoming more and more advantageous to capture images as colored digital images. Colored digital images are frequently stored in three color planes such as red, green, and blue, or cyan, magenta, and yellow. In image processing, these colored digital images luminance and and chrominance color coordinates are quite useful because they express color in a similar fashion to the way the human visual system operates. As is well known, luminance, the black and white portion of an image, determines the sharpness of such image while the chrominance values determines its colorfulness.
Color interpolation between pixels can reduce the noise level in the luminance channel, but not without the expense of increasing noise in the chrominance channel. As is also well known to those skilled in the art, generally three channels are used to describe a color. For example, if an image is recorded having red, green, and blue channels, this can be converted to one luminance channel and two chrominance channels such as Y, Cr, Cb. These luminance and two chrominance channels facilitate certain aspects of digital image processing.
In addition to accumulating noise in the chrominance channels, the process of color interpolation can also introduce interpolation error. Such effects are most likely to occur in regions of an image having both high contrast and high spatial frequency content. Stated differently, such regions of an image contain color values which change quickly and change by large amounts. The high degree of variability in all color values reduces their mutual spatial correlation which, in turn, undercuts the basis of the color interpolation algorithm and degrades its performance.
Furthermore, if the chrominance errors occur in a region where the true image content consists largely of neutral shades of gray, they will produce color speckles that are easily seen as objectionable image artifacts. Consequently, the most noticeable aspects of chrominance errors can be avoided by reducing chrominance in high contrast regions of an image.
It is an object of the present invention to correct for chrominance interpolation artifacts.
This object is achieved by a method for correcting chrominance interpolation artifacts in a digital color image having color pixels in which each colored pixel is expressed as one luminance and two chrominance color values, such method comprising the steps of:
a) computing a test value at each pixel which indicates the presence of a high contrast luminance feature; and
b) adjusting the chrominance values at pixels in accordance with the computed test value to correct for chrominance interpolation artifacts.
It has been determined that chrominance interpolation artifacts are highly correlated with rapidly changing luminance features. The present invention recognizes luminance Laplacians and adjusts chrominance values in accordance therewith. The chrominance values are, where appropriate, reduced and therefore the color content is desaturated depending upon the relative value of the computed test value and the luminance value.